Summary The Robinson R44 Raven II helicopter, N313AL (serial number10056), landed at the Cranbrook airport, British Columbia, at 1237 mountain standard time (MST), where the pilot filled the fuel tanks to capacity and obtained weather and flight-planning information. The helicopter departed Cranbrook at 1343MST for Revelstoke, evidently following the visual flight rules route north along the Columbia River, towards Fairmont Hot Springs. The flight was expected to take two hours. At 1415MST, the helicopter struck steep terrain 33nautical miles north-northwest of Cranbrook, at the 4200-foot level in a mountainous region. The pilot was fatally injured and the helicopter destroyed by impact forces and a severe post-crash fire. Ce document est galement disponible en franais. Other Factual Information The 1300 mountain standard time1 (2000Coordinated Universal Time) METAR2 for Cranbrook recorded cloud at 200feet above ground level (agl), with a measured ceiling at 1100feet agl; the wind was light from the west at 3knots and the temperature was -4C. At the time of the accident, the observed weather at the accident site was suitable for a visual flight rules flight, with an overcast layer at 5000feet above sea level, no precipitation, visibility of 5miles, and the temperature estimated at -12C. Robinson R44 Raven II, N313AL, was manufactured in March2003. A review of the logbooks and maintenance records indicates that the helicopter had been certificated, equipped and maintained in accordance with existing regulations and approved maintenance procedures. At the time of the accident, the helicopter had accumulated 343hours of flight time. The accident site is on steep mountainous terrain, surrounded by tall conifers. The helicopter first struck a tree in its downhill flight path and began to break up. It then continued in the air for a further 100feet, struck the ground at the base of another tree and rolled 25feet downhill, coming to rest against the bases of several other trees. The damage to the helicopter, trees and terrain indicate that the helicopter was in a steep descending flight-path when it first struck the trees, and then continued into a steep nose-down attitude as it entered the trees in a vertical profile. Wreckage information shows that the rotor was turning at first impact with the trees and more slowly at final impact with the terrain. To transfer engine power to the main transmission, the R44helicopter employs four double-vee drive-belts running around two sheaves - the lower "driving" sheave and the upper "driven" sheave. Engaging (or disengaging) the vee-belts is accomplished by the pilot raising (or lowering) the upper sheave using an electric actuator. In flight, the actuator unit senses and maintains vee-belt tension. In this accident, one of the four vee-belts was damaged, but intact, and found around the upper and lower shafts. A second vee-belt was found broken and was trapped in the wreckage between the upper sheave and the tail boom structure. No trace was found of the other two vee-belts. The eight grooves of the upper sheave had been partly coated with light soot from the post-impact fire. The eight grooves in the lower sheave contained rubber vee-belt residue. The four rear grooves had light vee-belt residue, but there was considerable rubber deposit on the four forward grooves. The paint primer surface of all eight grooves showed wear that was assessed as normal and acceptable. The consistency of the heavy rubber deposits on the forward grooves shows that the rubber transfer occurred at the same time for each vee-belt pair. The usual extension of the electric actuator that adjusts the vee-belt tension on the R44helicopter is between 0.8and 1.0inches; the accident actuator had a 1.3-inch extension. In the event that a vee-belt breaks during operation, the remaining vee-belts would carry the power transmission load between them, and the individual tensions would increase.3 This increase would be accomplished by the actuator operating (extending) until the total vee-belt tension was restored. Should a second vee-belt fail, the actuator would again operate to restore the total required tension. Controlled tests of vee-belt failure on an exemplar R44helicopter revealed that with two vee-belts removed, the actuator extended to 1.3inches to apply the necessary tension to maintain normal rotor rpm. The tests showed that the actuator takes 18seconds to extend from 1.0to 1.3inches. The engine installed in this R44 helicopter was a Textron Lycoming 6-cylinder reciprocating engine, modelIO-540-AE1A5 (serial number L-28579-48A), and it had been installed in N313AL at the time of manufacture in March2003. It had accumulated 343hours total since new (TSN). The post-crash examination of the engine showed that it was operating at moderate to high rpm at impact. A review of the engine maintenance records reveals that this engine had been removed from the helicopter at 299hours TSN on 25August2004, at the first 300-hour maintenance inspection, because of sticking exhaust valves in cylinder numbers3 and5, and spalling4 on the number5 exhaust valve tappet and cam. The engine was returned to Lycoming for inspection and repair, and was reinstalled in N313AL on 11November2004. The examination of the engine by the Transportation Safety Board of Canada (TSB) revealed that the ends of the intake valve stems all exhibited "lipping" damage in the keeper channel. This was caused by an engine overspeed event, after the inspection and repair at Lycoming, within the last 43hours. Furthermore, a microscopic examination of the damage suggested that the overspeed event occurred in the last minutes of engine operation. If one or more vee-belts broke, the sudden loss of belt tension would cause the remaining belts to slip on the driving sheave, in turn causing a rapid increase of engine rpm, likely an overspeed.5 Cylinder numbers 3 and 5,including new valves and valve guides, had been installed as-new by Lycoming 43flight hours before the accident. The post-accident examination of the exhaust valve guides for those cylinders revealed excessive wear for the time in service. The cause for such high wear could not be determined. The engine fuel control unit was examined and no anomaly was found. The throttle valve and metering orifice were in the full-open positions at impact, and the mixture mechanism was likely in the full-rich position. The engine cooling fan is attached to the engine by a taper-fit between the end of the drive shaft and the socket of the fan. Several technical reports6 record severe galling damage7 from in-flight usage, requiring rejection and replacement of the fan assembly. The effect of the galling on this joint introduces abnormal wear, imbalance and vibration, which in turn amplifies the galling mechanism and accelerates the wear process. The end result is a fan assembly that imparts increasing vibration to the lower sheave and the vee-belts. Vibration in the belt-drive system is one known factor in vee-belt failure, misalignment and loss.8 A metallurgical examination of both the tapered shaft and the tapered fan socket on the accident helicopter revealed remarkable galling, which was determined to have occurred within the last few hours of engine operation. Furthermore, the eight mounting bolts and washers, which attach the fan unit to the tapered socket, demonstrated remarkable fretting. The bolt holes in the fan backplate were elongated and the metal stretched at the inside diameter. None of this damage was present at the last maintenance inspection at 299hoursTSN. The TSB Engineering Branch analysis of the bolt/washer fretting and elongation revealed that the wear mechanism had occurred over a similar period to the galling of the taper-fit fan joint, and most likely concomitant with it. The stretching was assessed to have been caused by impact forces. TSB Engineering Branch completed LP021/05 - Fan Assembly and Drive Train Examination. This report is available from the Transportation Safety Board of Canada upon request.